Recombinant proteins provide effective therapies for many life-threatening diseases. The use of high expression level systems such as bacterial, yeast and insect cells for production of therapeutic proteins is mostly limited to small proteins without extensive post-translational modifications. Mammalian cell systems, can be used to produce proteins with many of the needed post-translational modifications, but can be more expensive due to the sophisticated culture systems that are, at times, required. Many of the limitations of mammalian cell culture systems have been overcome with the expression of recombinant proteins in the milk of transgenic mammals. Proteins have been produced in mammary glands of various transgenic animals with expression levels suitable for cost effective production at the scale of hundreds of kilograms of protein per year.